The present invention relates to a process for manufacturing gallium arsenide monolithic microwave integrated circuits wherein via holes can be placed in a thin gallium arsenide wafer while the wafer is adhesively held to a substrate, and the adhesive holding the wafer to the substrate can subsequently be easily removed.
Processes are well known in the semiconductor manufacturing art for etching via holes in GaAs wafers using either wet chemicals or dry plasma etching. In these processes wafers are thinned by lapping and chemical polishing to approximately 50 micrometers to 150 micrometers which insures that all of via holes are etched through to the front of the wafer, while none over etch to the extent that they etch out one or more of the front side contact pads causing a failure of the electrical continuity after subsequent back side metallization. Since via formation requires significant amounts of handling after wafer thinning, the known processes include the step of adhesively attaching the thinned GaAs wafer to a substrate. The adhesive agents used to attach the GaAs wafer to the substrate must be capable of withstanding all of the processing steps associated with via fabrication. Wax and photo resists have commonly been used as the adhesive agent in the prior art processes.
A major shortcoming of the aforementioned prior art processes is the difficulty of removing the adhesive agents from the front side of the gallium arsenide wafer after it has been processed. This problem is especially apparent where relatively thick wafers (125 micrometers to 150 micrometers) are used to minimize the loss of monolithic integrated circuits during production. The use of these relatively thick GaAs wafers requires dry etching to achieve the required via hole yield. However, the dry etch process heats the gallium arsenide which causes carbonization of waxes and polymerization of the photoresist. The chemically altered adhesives are then extremely difficult to remove from the front side of the GaAs wafer, and any unremovable residues render the integrated circuit useless.